An important aspect of environmental control is monitoring the Earth's atmosphere and water basins. Atmospheric aerosols that are contained in the Earth's atmosphere play important roles in climatology and visibility as they absorb and scatter solar radiation. They also may affect human health when they penetrate the human body via the respiratory tracts. Therefore, there have been increased efforts aimed at better characterization of chemical and microphysical properties of aerosols to help elaborate appropriate particulate matter emission standards. Understanding of properties and behavior of atmospheric aerosols is also extremely important for studying the Earth's climate and potential detrimental impact of the aerosols on air quality and human health.
Control of water consists of flow routing along the river network, especially in connection with human activity, surveying of hydrological processes of land-atmospheric interaction such as evapotranspiration and snowmelt, control of sediment and pollutant transport in the streams, etc. It is not less important to control the pollution of water in seas and oceans, especially in the populated coastal areas. The protection of the water supplies is an important goal also for Homeland Defense to prevent a pandemic disaster. A future terrorist tactic could include dispersing of the poison-containing ampoules that can be triggered by remote control. The ampoules could be dispersed in air as aerosols or moved invisibly underwater and put in the bottom of the reservoir.
An instrument, which is normally used, for controlling environmental conditions of water and gases is an aerosol mass spectrometer. Irrespective of whether the samples are taken from water or air, a mass spectrometer per se operates with dry particles or dried particles. In the case when samples are taken from water, prior to admission into the vacuum chamber of the mass spectrometer, the samples are pretreated to form a stream of dried discrete particles. The samples are dried even if they are taken from moisture-containing air. Since the present invention relates to an aerosol mass spectrometer and since the particles or particles enter the mass spectrometer already in a dry state, the following analysis of the prior art will relate merely to aerosol mass spectrometers without distinction between those taking samples from water or the atmosphere.
A typical aerosol mass spectrometer consists of the following parts: a sample inlet unit with a system for preparation and introduction of a substance to be analyzed into the instrument; a source of particles; an ionization device where the aforementioned particles are charged and formed into an charged particle flow; a mass analyzer where the charged particles are separated in accordance with an M/Z ratio, focused, and are emitted from the particle source in various directions within a small space angle; a charged-particle receiver or collector where current of charges is measured or converted into electrical signals; and a device for amplification and registration of the output signal. In addition to amount of charged particles (ion current), the registration unit also receives information about charged particle mass. Other units included into a mass spectrometer are power supplies, measurement instruments, and a vacuum system. The latter is required for maintaining the interior of the mass spectrometer under high vacuum, e.g. of about 10−3 to 10−7 Pa. Operation is normally controlled by a computer, which also stores the acquired data. According to common understanding, ions are defined as charged atoms or molecules of a substance. However, since the aerosol mass spectrometer of the present invention works not only with ions but also with larger particles that may be aggregated from thousands or more than thousands of molecules, where appropriate, instead of the word “ion”, we will use the word “particle” which covers both the ions and particles larger than ions. In some instances, the word “ion” will be still used in compliance with the generally used terminology. For example, the word “ion” is present in the words “ionizer” and “ionization”.
The particles contain organic and inorganic compounds and elemental carbon black, graphite-like material. The particle-phase compounds can be divided into primary and secondary. The primary particulate compounds are of a particle origin, while the secondary compounds results from emission of gases, which then underwent chemical transformation in the atmosphere and condensed on the pre-existing particles. Primary and secondary compounds are emitted by both natural (sea salt from oceans, isoprene from plants) and anthropogenic sources (soot and organics from combustion sources, ammonia from cattle feedlots, etc.). Whether the gas-phase organics are natural or anthropogenic, many can react photochemically in the atmosphere usually by one of three paths: Photons cleave a bond, OH radicals abstract a hydrogen, or ozone reacts with a carbon-carbon double bond. This initial step is often followed by a chain of rapid reactions until a more stable molecule results. Reactions with ozone often produce oxygenated compounds with much lower vapor pressures than the parent compound. That is, the parent compound had a high vapor pressure so was in the gas phase. The daughter compound has a lower vapor pressure so condenses on pre-existing particles forming SOA (Secondary Organic Aerosol). The lower vapor pressure often comes from a compound that became water soluble (polar).
In fact, the aforementioned microparticles may appear to be extremely dangerous even if their concentration in air or water is insignificant. A good example of such dangerous substances is organic poison, such as ricin, or the like. Such dangerous substances in small concentrations can be localized in small areas, which are separated by vast non-polluted spaces, and, in order to detect the presence of such substances, it is necessary to scan these vast spaces with high rate and in a continuous high-duty mode of analysis, in order not to miss the aforementioned areas of poison localization.
In view of the above, there is a demand for development of real-time aerosol detection methods and apparatuses that would allow not only estimate mass and speciation of organic matter as a function of size but also quickly evaluate the photochemical evolution of the organic aerosol or identify isotopic and molecular-level tracers of primary and secondary organic carbon in a continuous high-duty mode of analysis.
A mass spectrometer is characterized by its resolution capacity, sensitivity, response, and a range of measured masses. The aforementioned response is a minimal time required for registration of mass spectrum without the loss of information within the limits of so-called decade of atomic mass units (1–10, 10–100, etc.). Normally such time is 0.1 to 0.5 sec. for static mass spectrometers and 10−3 for dynamic (time-of-flight) mass spectrometers.
A substance to be analyzed is introduced into the mass spectrometer with the use of so-called molecular or viscous flow regulators, load ports, etc.
By methods of ionization, ion sources of mass spectrometers can be divided into various categories, which are the following: 1) ionization caused by collisions with electrons; 2) photo-ionization; 3) chemical ionization due to ionic-molecular reactions; 4) field ion emission ionization in a strong electric field; 5) ionization due to collisions with ions; 6) atomic-ionization emission due to collisions with fast atoms; 7) surface ionization; 8) spark discharge in vacuum; 9) desorption of ions under effect of laser radiation, electron beam, or products of decomposition of heavy nuclei; and 10) extraction from plasma.
In addition to ionization, in mass spectrometer an ion source is used also for forming and focusing an ion beam.
More detail general information about types and constructions of ion sources suitable for use in mass spectrometers can be found in “Industrial Plasma Engineering” by Reece Roth, Vol. 1, Institute of Physics Publishing, Bristol and Philadelphia, 1992, pp. 206–218.
By types of analyzers, mass spectrometers can be divided into static and dynamic. Static mass spectrometers are based on the use of electric and magnetic fields, which remain, during the flight of charged particles through the chamber, practically unchanged. Depending on the value of the M/Z ratio, the charged particles move along different trajectories. More detailed description of static and dynamic mass spectrometers is given in pending U.S. patent application Ser. No. 10/058,153 filed by Yu. Glukhoy on Jan. 29, 2002.
It should be noted that static mass spectrometers are static installations, which are heavy in weight, complicated in construction, and operation with them requires the use of skilled personnel.
In time-of-flight mass spectrometers, charged particles formed in the ionizer are injected into the analyzer via a grid in the form of short pulses of charged-particle current. The analyzer comprises an equipotential space. On its way to the collector, the pulse is decomposed into several sub-pulses of the charged-particle current. Each such sub-pulse consists of charged particles with the same e/m ratios. The aforementioned decomposition occurs because in the initial pulse all charged particles have equal energies, while the speed of flight V and, hence, the time of flight t through the analyzer with the length equal to I are inversely proportional to m1/2.T=L(m/2 eV)1/2.
A series of pulses with different e/m ratios forms a mass spectrum that can be registered, e.g., with the use of an oscilloscope. Resolution capacity of such an instrument is proportional to length L.
An alternative version of the time-of-flight mass spectrometer is a so-called mass-reflectron, which allows an increase in resolution capacity due to the use of an electrostatic mirror. Energies of charged particles collected in each packet are spread over the temperature of the initial gas. This leads to broadening of peaks on the collector. Such broadening is compensated by the electrostatic mirror that prolongs the time of flight for slow charged particles and shortens the time of flight for fast charged particles. With the drift path being the same, the resolution capacity of a mass reflectron is several times the resolution capacity of a conventional time-of-flight mass spectrometer.
In the charged particle source of an RF mass spectrometer, charged particles acquire energy eV and pass through a system of several stages arranged in series. Each stage consists of three spaced parallel grids. An RF voltage is applied to the intermediate grid. With the frequency of the applied RF field and energies eV being constant, only those charged particles can pass through the space between the first and intermediate grids that have a predetermined M/Z ratio. The remaining charged particles are either retarded or acquire only insignificant energies and are repelled from the collection by means of a special decelerating electrode. Thus, only charged particles with the selected M/Z ratio reach the collector. Therefore, in order to reset the mass spectrometer for registration of charged particles with a different mass, it is necessary either to change the initial energy of a flow of charged particles, or frequency of the RF field.
Magnetic resonance mass analyzers operate on a principle that the time required for ions to fly over a circular trajectory will depend on the ion mass. In such mass analyzers, resolution capacity reaches 2.5×104.
The last group relates to ion-cyclotron resonance mass spectrometers in which electromagnetic energy is consumed by charged particles, when cyclotron frequency of the charged particles coincides with the frequency of the alternating magnetic field in the analyzer. The charged particles move in a homogeneous magnetic field B along a spiral path with so-called cyclotron frequency ωc=eB/mc, where c is velocity of light. At the end of their trajectory, the charged particles enter the collector. Only those charged particles reach the collector, the cyclotron frequency of which coincides with that of the alternating electric field in the analyzer. It is understood that selection of charged particles is carried out by changing the value of the magnetic field or of the frequency of the electromagnetic field. Ion-cyclotron resonance mass spectrometers ensure the highest resolution capacity. However, mass spectrometers of this type require the use of very high magnetic fields of high homogeneity, e.g., of 10 Tesla or higher. In other words, the system requires the use of super-conductive magnets, which are expensive in cost and large in size.
In a quadrupole mass spectrometer, charged particles are spatially redistributed in a transverse electric field with a hyperbolic distribution of the electric potential. This field is generated by a quadrupole capacitor having a D.C. voltage and RF voltage applied between pairs of rods. The flow of charged particles is introduced into a vacuum chamber of the analyzer in the axial direction of the capacitor via an input opening. With the frequency and amplitude of the RF field being the same, only charged particles with a predetermined M/Z ratio will have the amplitude of oscillations in the transverse direction of the analyzer shorter than the distances between the rods. Under the effect of its initial velocity, such charged particles will pass through the analyzer and will be registered and reach the collector, while all other charged particles will be neutralized on the rods and pumped out from the analyzer. Reset of such mass spectrometer to charged particles of another mass will require to change ether the amplitude or the frequency of the RF voltage. Quadrupole mass spectrometers have resolution capacity equal to or higher than 103.
Attempts have been made to improve existing mass spectrometers of the time-of-flight type, e.g., by improving charged-particle storage devices, introducing deflectors for selection of charged-particle for analysis in a mass spectrometer, reorganizing sequencing of charged-particle packets or by extending the time of flight for improving resolution capacity of the mass spectrometers.
For example, U.S. Pat. No. 5,396,065 issued in 1995 to C. Myerholtz, et al. discloses an encoded sequence of charged-particles in packets for use in time-of-flight mass spectrometers, in which the high-mass charged particles of a leading packet will be passed by the low-mass charged particles of a trailing packet. Thus, a high efficiency time-of-flight mass spectrometer is formed. The charged particles of each packet are acted upon to bunch the charged particles of the packet, thereby compensating for initial space and/or velocity distributions of charged particles in the launching of the packet. The times of arrival of the charged particles are determined at the detector to obtain a signal of overlapping spectra corresponding to the overlapping launched packets. A correlation between the overlapping spectra and the encoded launch sequence is employed to derive a single non-overlapped spectrum.
However, such method and apparatus make interpretation of obtained data more complicated and not easily comprehensible. Furthermore, addition electronic circuits are required for control of the charged particle packet sequence.
U.S. Pat. No. 5,753,909 issued in 1998 to M. Park et al. describes a method and apparatus for analyzing charged particles by determining times of flight including using a collision cell to activate charged particles toward fragmentation and a deflector to direct charged particles away from their otherwise intended or parallel course. A disadvantage of this device consists in that it is based on the selection of specific charged particles and does not show the entire mass spectrum. For obtaining the entire spectrum, it is necessary to perform step by step scanning, and this requires an additional time.
A disadvantage of the device disclosed in U.S. Pat. No. 5,753,909 consists in that this mass spectrometer is based on the selection of specific charged particles and does not show the entire mass spectrum. For obtaining the entire spectrum, it is necessary to perform step by step scanning, and this requires an additional time.
U.S. Pat. No. 6107,625 issued in 2000 to M. Park discloses a coaxial multiple reflection time-of-flight mass spectrometer of a time-of-flight type with resolution capacity improved due to a longer time of flight of the charged particles. The apparatus comprises two or more electrostatic reflectors positioned coaxially with respect to one another such that charged particles generated by a charged-particle source can be reflected back and forth between them. The first reflecting device is a charged-particle accelerator which functions as both an accelerating device to provide the initial acceleration to the charged particles and a reflecting device to reflect the charged particles in the subsequent mass analysis. The second reflecting device is a reflectron, which functions only to reflect the charged particles in the mass analysis. During the mass analysis, the charged particles are reflected back and forth between the accelerator and reflectron multiple times. Then, at the end of the charged-particle analysis, either of the reflecting devices, preferably the charged-particle accelerator, is rapidly de-energized to allow the charged particles to pass through that reflecting device and into a detector. By reflecting the charged particles back and forth between the accelerator and reflectron several times, a much longer flight path can be achieved in a given size spectrometer than could otherwise be achieved using the time-of-flight mass spectrometers disclosed in the prior art. Consequently, the mass resolving power of the time-of-flight mass spectrometer is substantially increased.
This is a typical system with storage of charged particles, which does not allow a continuous mode of mass analysis since it requires some period for de-energization of one of the reflecting devices. Obviously, the data is difficult to interpret, especially when masses of charged particles are scattered in a wide range so that light charged particles may undergo several reflections while heavy charged particles made only one or two reflections.
The most advanced time-of-flight mass spectrometer (TOF MS) that provides extended time of flight trajectory and hence the time resolution is a quadrupole mass spectrometer developed by Y. Glukhoy and described in aforementioned U.S. patent application Ser. No. 10/058,153. This is the first mass spectrometer known in the art that provides helicoidal trajectories of charged particles by using only electrostatic lens optics.
A mass spectrometer of the aforementioned patent application is based on the use of quadrupole lenses with an angular gradient of the electrostatic field from lens to lens. The device consists of a charged-particle source connected to a charged-particle mass separation chamber that contains a plurality of sequentially arranged electrostatic quadrupole lenses which generate a helical electrostatic field for sending charged particles along helical trajectories in direct and return paths. Scattering of positions of points of return is reduced by means of electrostatic mirrors located at the end of the direct path, while charged particles of different masses perform their return paths along helical trajectories different from those of the direct paths due to the use of a magnetic and/or electrostatic mirrors.
A particle-electron emitting screen is installed on the path of charged particles in the return path, and positions of collision of the charged particles with the particle-electron emitting screen over time and space are detected with the use of micro-channel plate detectors. Movement of charged particles along the helical trajectory significantly increases the path of charged particles through the charged-particle separation chamber and, hence, improves the resolution capacity of the mass spectrometer.
However, the above-described helical-path quadrupole mass spectrometer, as well as all aforementioned known mass spectrometers of other types, is not very convenient for aerosol applications. This is because in some applications the aerosol analysis should be carried out with sampling and inputting of the aerosol substance into the mass-analyzing unit in a continuous mode. At the same time, all aforementioned apparatuses have a low-duty cycle and are characterized by a limited particle input, i.e., they have a single injection port for inputting particles to be analyzed into the mass spectrometer.
It should be noted that the use of mass spectrometers has come under scrutiny in recent years as a possible solution for a high-speed detection of the aerosol particles in the panorama mode. It can be used for early detection and real-time analysis of aerosol particles in the situation of the large area contamination after the chemical and biological attack or accident, or for general-purpose field, e.g., for monitoring of ozone-consuming organic materials, or the like.
However, the sensitivity of conventional TOF MS is affected by the aforementioned low-duty cycle, meaning only small fraction of charged particles originally in the continuous flow of charged particles is converted into the charged-particle packets and participates in the registration by the charged-particle detectors. Most of the charged particles are discarded from registration during “pulse and wait” time.
It should be recalled that an aerosol TOF MS is supposed to combine several processes which are the following: collection and preparation of samples to a form acceptable for mass spectroscopy; electron impact ionization; bunching of charged particles upon application of an electrical pulse to the gating electrode (usually a charged grid) i.e., conversion of the continuous flow of charged particles into the charged-particle packets; collimation of the flow of charged particles by introducing these charge-particle packets into the charged-particle flight region; traveling of the charged particles in the long drift tube; detecting the charged particles impinging the multi-channel plates; and analyzing the obtained data.
In all known aerosol TOF MS's, a significant amount of sample material is wasted. Usually 98% of the sample is lost during passing through the nozzle, skimmer's collimation, electron impact ionization and the entrance aperture. These losses are unavoidable. But others can be reduced significantly. For example, traveling losses due to collisions with molecules of the residual gas can be reduced by improving the vacuum and reducing the length of the drift tube. This objective was achieved in aforementioned U.S. patent application Ser. No. 10/058,153 due to the use of an extended (direct-return) helical trajectory of the particles.
It should be noted, that analysis conducted in a conventional aerosol TOF MS requires that the continuous flow of particles be interrupted. Otherwise, it would be impossible to perform selection and tracing of individual particles for which the time-of-flight and, respectively, spectra of masses, have to be determined. However, in conventional aerosol TOF MS, bunching, i.e., in a process that extracts particles from a continuous charged-particle flow, is insufficient and therefore in some cases leads to the loss of very important information and hence to decrease in the sensitivity of the TOF-MS as whole. To increase the signal-to-noise ratio, such conventional systems use expensive amplifiers and logistical systems.
Conventionally, the stream of charged particles is divided into packets of ions that are launched along the propagation path using a traditional “pulse-and-wait” approach. The second packet can't be launched before all charged particles from the first packet reach the charged-particle detector in order to prevent overlapping of signals. Because each packet can contain only a few charged particles of the species of the materials, the experiment has to be repeated many times. So, it is impossible to reach in the condition of the flight the quality of the measurement that is sufficient to identify the aerosol compound using a conventional TOF MS. In other words, conventional TOF MS's have a limited low-duty cycle, and the authors are not aware of any known means that can increase the duty cycle above 60%.
For measurement of masses of particles, the data obtained in an aerosol TOF MS must be analyzed. Heretofore, different methods have been used for reconstruction of the particle distribution spectra in acquisition period of the cycle. Such methods are described e.g., by the following authors: 1) G. Wilhelmi, et al. in “Binary Sequences and Error Analysis for Pseudo-Statistical Neutron Modulators with Different Duty Cycles,” Nuclear Inst. and Methods, 81 (1970), pp. 36–44; 2) Myerholtz, et al. “Sequencing ion packets for ion time-of-flight mass spectrometry” (see aforementioned U.S. Pat. No. 5,396,065 described earlier in the description of the prior art); 3) Cocg “High-duty cycle pseudo-noise modulated time-of-flight mass spectrometry” (U.S. Pat. No. 6,198,096, issued Mar. 6, 2001; 4) Brock, et al. “Time-of-flight mass spectrometer and ion analysis” (U.S. Pat. No. 6,300,626, issued Oct. 9, 2001); 5) Overney, et al. “Deconvolution method and apparatus for analyzing compounds” (U.S. Pat. No. 6,524,803, issued Feb. 25, 2003), etc.
The above methods utilize special properties of the pulsing sequence, e.g., a pseudo-random binary sequence (PRBS) or Hadamard Transform. However, they cannot reach a high-duty cycle because their TOF MS's annihilate a part of the flow of charged particles by a gating grid [see references 3) and 4)] or deflecting mesh [see reference 5)] during binary modulation that they converted. This is because at least a half of the charged-particle flow must be discarded to allow the other half to be counted. The flow of charged particles sputters and contaminates the modulation grids or meshes and creates secondary electron-, ion-, or photon-emission leading to deterioration of the grids. Furthermore, foreign species introduced in the drift space because of contamination and sputtering destruct the detectors and distort the information. The low sensitive flat deflection system, which is used in the in the A. Brock et al TOF-MS for the Hadamard's transform, contains a high density array of the wires with alternating potential that leads to breakdown.
So the conventional TOF-MS's with the pseudo-random binary methods of bunching of the ion packets can not provide high-duty cycle, have low sensitivity and reliability, and cannot serve properly as monitoring devices for field applications because of the incorrect choice and design of the ion optics and the irrational bunching strategy.
The disadvantages of the known aerosol TOF MS's make them unsuitable for aforementioned high-duty analysis under extreme or critical conditions such a biological attack or an environmental disaster, e.g., a hazardous leakage or contamination of water reservoirs in populated areas.